Fix MMCV_CUDA_ARGS so it supports multiple arch

[larroy]

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Motivation

Currently is not possible to support multiple cuda arches because a space separated string of gencode arguments get passed as a single nvcc argument which triggers a compile error (should be separate arguments).

Modification

Split the MMCV_CUDA_ARGS variable to correctly pass it to the compiler.

BC-breaking (Optional)

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If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

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Checklist

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